While neural command systems are thought to play an important role in organizing the behavior of animals, little is known about the natural excitatory inputs to most command systems or about inhibitory interations among them. The long-term objectives of this research are (i) to describe how identified sensory neurons in crayfish excite identifiable command neurons (CNs) that release abdominal movements and (ii) to describe how inhibitory interactions among command systems prevent the simultaneous release of mutually exclusive behavioral responses. Descriptions of these phenomena should provide insights into the neural bases of behavioral choice in this and other animals. The sensory excitation is provided by a set of central nervous photoreceptors, the caudal photoreceptor neurons (CPRs), which excite command interneurons in the rostral nervous system. These CNs project caudally to release backware walking and abdominal postural movements and to inhibit other command systems, including those for escape. Escape CNs inhibit the abdominal postural motoneurons and may inhibit abdominal postural CNs. These inhibitory interactions among central command systems appear to allow the animal to do one thing at a time. The proximate goals of this application are: (1) to identify abdominal postural CNs in the rostral CNS that are excited by the CPR neurons; (2) to determine the anatomical relationship between the CPR terminals and the input regions of the command cells; (3) to record the spike and synaptic responses of those command cells to CPR spike trains; (4) to describe the interactions between CPR and other afferent inputs to the CNs; and (5) to describe the inhibitory relations between abdominal postural CNs and escape command cells. Extracellular recording and stimulating techniques will be used to identify the axons of abdominal postural command interneurons within interganglionic connectives. Responses of CNs to electrical CPR stimulation will be recorded alone and during sensory stimulation that normally excites or inhibits those neurons. Responding CNs will be stained and their effects on postural motoneurons recorded. Both CPRs and the CNs they excite will be traced electrophysiologically and anatomically to their site of interaction in the rostral CNS. The command neurons will be penetrated and their synaptic responses to CPR activity recorded. Synaptic inputs from other afferents and command interneurons will also be recorded.